1. Field of the Invention
The present invention relates generally to an apparatus for shaping a curved tempered glass sheet for use as a front windshield for automobiles, for example, and more particularly to an improved ring mold unit for shaping and tempering a glass sheet in such an apparatus.
2. Description of the Relevant Art
Systems for shaping glass sheets for use as curved automotive window glass sheets, for example, operate by heating a flat glass sheet in a heating furnace up to the softening point of glass, feeding the heated glass sheet with a plurality of conveyor rollers in the heating furnace to a position in a shaping device, bending the glass sheet with upper and lower mold members in the shaping device, and then applying a suitable cooling medium to the curved glass sheet to temper the same. It has been customary to use a ring mold for supporting the peripheral edge of the glass sheet to bend the same with increased accuracy. When the bent glass sheet is quenched in the shaping device, the glass sheet is also supported by the ring mold. With the lower surface of the glass sheet being supported by the ring mold at the time of quenching the glass sheet, however, cooling air tends to remain undischarged between the glass sheet and the ring mold without sufficiently reaching the peripheral edge of the glass sheet, thus failing to temper the glass edge.
In view of the above shortcoming, there has been proposed a ring mold having a number of recesses defined in the upper surface thereof for allowing cooling air to be supplied in the vicinity of the peripheral edge of a glass sheet to be quenched. The proposed ring mold, however, raises another problem in that the recessed surface of the ring mold is apt to leave depressions and projections on the lower surface of the shaped glass sheet.
Japanese Laid-Open patent publication No. 57/145041 shows a ring mold unit comprising a cooling ring mold having a number of recesses and a shaping ring mold disposed outwardly of the cooling ring mold. After a glass sheet is bent by the shaping ring mold, cooling air is applied against the glass sheet while it is being supported by the cooling ring mold, thereby tempering the glass sheet. Since the shaping ring mold is constructed to support only the peripheral edge of the glass sheet, the surface area of the glass sheet which is supported by the shaping ring mold is relatively small, and hence mechanical distortions are liable to occur in the peripheral edge of the glass sheet. Moreover, the cooling mold leaves projections and depressions on the area of the glass sheet which is held by the cooling ring mold, and the peripheral edge of the glass sheet which is supported by the shaping ring mold cannot be well tempered inasmuch as the glass edge is not sufficiently cooled.
To eliminate the above drawbacks, there has been proposed a glass sheet shaping ring mold having a web of metallic woven cloth made of stainless steel fibers placed on the recessed surface of the ring mold which is disposed for contact with a glass sheet, as disclosed in Japanese Laid-Open patent publication No. 59/149934. The metallic woven cloth prevents the glass sheet from being held in direct contact with the ring mold that is held at a low temperature, and also prevents depressions and projections from being left on the glass sheet. However, cooling air is not well discharged from between the glass sheet and the ring mold, so that the glass sheet will not be tempered as well at the peripheral edge as at the other area thereof.